Newbie needs help with custom phantom circuit

Hello all,

Newbie here, Just finished my year long course in electronics so now I want to get into the nuts and bolts of electronics in audio.

For my first project I would like to build a passive DI/headphone amp box.

I want to try to supply power to the headphone amp using the same mic cable the will be coming out of the DI section. I am going to build a separate power supply for the amp which requires +/- 12 V 500mA and I want to somehow be able to feed this down the cable using the phantom power idea, but of course not actually using the phantom power from the mixer but my own power supply.

I have had a play around in multisim and i can get +12V down the cable using 24 ohm resistors, but given my lack of theory I cant seem to be able to tap into the 12V off the cable without causing the audio to dropout. I've used 400uF capacitors on each end. I'm thinking i could perhaps send 24V down the cable and somehow be able to tap this 24V supply and convert it to +/- 12 V without messing up the audio ? Is this even possible given i need around 500mA

I think i just need more theory in how the dc in the cable gets tapped correctly without affecting the AC

If anyone could point me in the right direction  I would be most grateful.

Nah, you can't do that because you need resistors large enough not to load the source but pass enough current to power the device. But a headphone amp could pull 50+ mA which is going to cause a massive drop in supply voltage. Use ohms law to understand how much / what the relation is.

If the cable is not long, you don't need balanced but you could still use the two conductor cable using one for signal and the other for power.

damo76 said:

For my first project I would like to build a passive DI/headphone amp box.
I want to try to supply power to the headphone amp using the same mic cable the will be coming out of the DI section. I am going to build a separate power supply for the amp which requires +/- 12 V 500mA and I want to somehow be able to feed this down the cable using the phantom power idea, but of course not actually using the phantom power from the mixer but my own power supply.

Click to expand...

It looks IMO too complex for a first project. But,

.... to tap this 24V supply and convert it to +/- 12 V without messing up the audio ? Is this even possible given i need around 500mA

Click to expand...

Headphone amps can be supplied with a single power supply. 24V for example. Why do you need 0.5 A? How many headphones will be connected to one box?  10? One or two headphones per box  is a more realistic situation.

I think i just need more theory in how the dc in the cable gets tapped correctly without affecting the AC

Click to expand...

It is possible to design it (IMO not for more than two headpohones, 600ohms types if possible), keywords are
#inductor  and #CCS (constant current source)

The headphone circuit is an ebay kit. I've built one before and they require +/-12 V. As soon as the next one arrives I will drive it hard and check how much current it uses. The DI circuit is just a simple passive job.

I've changed my approach a bit. I'm going to feed the DC into the output lines going TO the amp. The left channel i'll pass (+12V) and the right channel I'll pass (-12V).    I've pasted what I've designed so far. I just found more theory and so I have managed to find out how to tap off using resistors. Any suggestions fully welcome. I'm on a fast learning curve

Cheers

Also, according to the datasheet for the TPA6120 IC (amp chip), the Ic is 13mA and the Oc is 700mA . Only will be used for 1 headphone and would like ample headroom to drive the headphones loud. (for a drummer). 

damo76 said:

For my first project I would like to build a ... headphone amp box.

I want to try to supply power to the headphone amp using the same mic cable the will be coming out of the DI section.

Click to expand...

Unfortunately you have chosen a subject that even experienced designers will find difficult. Phantom powering generally results in low efficiency, because one has to start with a significantly higher voltage than what is strictly needed, and agree to waste a large part of this voltage in voltage-dropping resistors. If I had to do it, I really would simply build a standar h/p amp and rely on cables to deliver to the phones.
Now if there is a strong reason to have the amp near the h/p and no PSU near, there are solutions, but I think you would need to use a single-rail solution.
Look at page 8 of https://www.manualslib.com/manual/327144/Rts-Bp320.html?page=8#manual for how the single-rail amp gets its power from a rail that has audio superimposed on it.

As you have already been warned this is not simple, or very practical. Of course anything can be done with enough effort, but not likely worthwhile unless there is some extenuating circumstance driving this approach over simply separate power and audio.

I recall decades ago a manufacturer tried to use phantom powered (small) IC amps colocated at each speaker for the instal market. This was a solution for something the industry did not consider a problem so never gained traction in that market.

JR

Transformers?